The invention relates in general to a digital to analog converter, and more particularly to a current-driven thermometer code digital to analog converter.
Organic light emitting diode (OLED) displays are among currently prevailing flat panel displays. Since the brightness of an OLED is proportional to the current conducted thereby, variations of current level have great impact on uniformity of an OLED display, and improvement of a current-driving structure increases display uniformity. Thus, the quality of a current-driven digital to analog converter is critical to current output and display quality.
A thermometer code digital to analog converter is current driven and has advantages such as fast response and accurate current output. The operational duration of current sources in a conventional structure, however, differ.
FIGS. 1A˜1D are illustrations of current outputs of a 4-bit thermometer code digital to analog converter. The digital input data of FIGS. 1A˜1D are (0000), (0001), (1000) and (1111), respectively. The shadowed current sources are turned on to output current. From FIGS. 1A˜1D, it is found that a current source 101 is always on except in the case where the digital input data is (0000). To the contrary, a current source 116 is always off except in the case where the digital input data is (1111). In a conventional structure of a current driver, a thin film transistor in the current cell 101 is often influenced by current stress. This results in device degradation and thermometer code digital to analog converter malfunctions.
FIG. 2A shows a conventional 6-bit thermometer code digital to analog converter. The thermometer code digital to analog converter includes digital data input B1˜B6, a column decoder 202, a row decoder 204 and a current cell array 206. The digital data input B1˜B6 represent the least significant bit (LSB) to the most significant bit (MSB) of the 6-bit digital input data, respectively. The column decoder 202 decodes the bits B1˜B3 and outputs signals C1˜C7. The column decoder 204 decodes the bits B4˜B6 and outputs signals R2˜R8. R1 is a fixed high voltage level while C8 and R9 are fixed at a low voltage level. The current cell array 206 includes 64 current cells A(1,1)˜A(8,8). Each of the current cells A(1,1)˜A(8,8) includes a current mirror CMR and a logic gate LG, as shown in FIG. 2B. The logic gate LG receives one of the signals C1˜C8, represented by C_N, and two of the signals R1˜R9, represented by R_N and R_N+1, and afterward outputs a logic signal to control ON/OFF of the current mirror CMR.
Take a 6-bit digital input data (011110) as an example. Operation of the current cell array 206 is shown in FIG. 2A. Thirty ones of the current cells, shadowed in the figure, are turned on, while others do not output current. From the explanation of FIG. 1, the current cell A(1,1) is the most one often turned on. In other words, the current cell A(1,1) is often influenced by a current stress. This results in degradation of device characteristics and an abnormal current output.